Referring to FIGS. 5 and 6, a conventional model engine 1 contains a piston chamber 2, a crank chamber 3, an inlet 4, and an exhaust port 5. The piston chamber 2 has a piston 6 and a connecting shaft 7. The crank chamber 3 has a crankshaft 8, and the crankshaft 8 has a cam disc 8a with a shank 8b so as to drive the connecting shaft 7 to rotate. The crankshaft 8 has a window 8c defined on a middle section thereof and a channel 8d defined therein and passing an end surface of the cam disc 8a. Thereby, when the model engine operates, mixed oil gas flows into the crank chamber 3 from the inlet 4 via the window 8c and the channel 8d so that the piston 6 and the crankshaft 8 are driven to operate by the connecting shaft 7, and then a pressure generates above the piston chamber 2 so as to push the piston 6 to move downwardly, thereafter an orifice 2a turns on so that the mixed oil gas in the crank chamber 3 are drawn into the orifice 2a by the piston 6 and flows above the piston chamber 2, thus burning and discharging exhaust gas out of the exhaust port 5. However, when the crankshaft 8 rotates to push the mixed oil gas into the crank chamber 3, the piston chamber 2 generates a damp force, thus decreasing output power.
The present invention has arisen to mitigate and/or obviate the afore-described disadvantages.